As shown by Heath et al. U.S. Pat. No. 3,847,867, incorporated herein by reference and assigned to the same assignee as the present invention, polyetherimides are available which are valuable high performance injection moldable thermoplastic materials. Although commercially available polyetherimides have been found to have superior solvent resistance and a glass transition temperature (T.sub.g) of about 217.degree., these injection moldable materials cannot be employed in a variety of applications requiring enhanced solvent resistance and heat distortion temperatures.
Efforts have been made to enhance the T.sub.g and the solvent resistance of polyetherimides resulting from the intercondensation of BPA dianhydride (BPADA) and aryldiamine, for example meta-phenylene diamine (MPD). One procedure, for example, is by using BPADA with another dianhydride, such as pyromellitic diahydride (PMDA) or thioether dianhydride (SDA) and thereafter intercondensing the dianhydride mixture, either jointly or separately with aryldiamine, as shown by Williams et al., U.S. Pat. No. 3,983,093, assigned to the same assignee as the present invention and incorporated herein by reference. As taught by Williams et al., polyetherimides resulting from the use of BPADA with other dianhydrides have improved solvent resistance and higher T.sub.g 's, as compared to the polyetherimides resulting from the intercondensation of BPADA and aryldiamine. However, it has been found that polyetherimide films containing greater than about 20 mole percent of SDA based on the total moles of BPADA and SDA are often brittle and opaque. It has also been found that polyetherimides made from mixtures of BPADA and PMDA often contain unwanted block copolymer formation due to the differences in the reactivity of the respective dianhydrides during intercondensation. In addition, polyetherimide resulting from the intercondensation of mixtures of BPADA and PMDA also can be extremely rigid and become unprocessable in instances where the PMDA exceeds 20 mole percent of the dianhydride mixture based on the total moles of BPADA and PMDA.
The present invention is based on the discovery that polyetherimide copolymers resulting from the intercondensation of aryl diamine with mixtures of BPADA and oxydiphthalic anhydride, (ODAN), which hereinafter means 4,4'-oxybisphthalic anhydride, 3,4'-oxybisphthalic anhydride, 3,3'-oxybisphthalic anhydride, or mixtures thereof and preferably 4,4'-oxybisphthalic anhydride or a mixture of 4,4'-oxybisphthalic anhydride and up to 50 mole percent of 3,4'-oxybisphthalic based on total moles of 4,4' and 3,4', can result in the production of tough, flexible, solvent resistant transparent films having T.sub.g 's in the range of from about 225.degree. to 299.degree. C.